A video conference usually involves multiple parties. In the past, only the picture of one of the rest parties in the video conference could be seen by the local party because multi-picture technique was unavailable, which degraded intuitiveness of the video conference to a certain extent and made any party in the video conference feel that the communication with all other parties was not face-to-face. Therefore, it is very important to implement multi-picture technique that enables display of pictures of multiple parties on a display screen, in order to improve the performance of videoconferencing and meet the users' demand.
In the prior art, the multi-picture transmission solution usually includes either of the following two methods:
Method 1: First, the Multipoint Control Unit (MCU) decodes video bitstreams from each terminals to obtain the corresponding multiple channels of decoded pictures; second, the decoded pictures are scaled to appropriate sizes and combined together; next, the combined picture is encoded and the resulting bitstream is transmitted to a terminal; finally, the terminal decodes the combined picture bitstream and the decoded pictures are the multi pictures.
Method 2: The MCU performs multiplexed transmission of the multiple channels of video bitstreams on the basis of Continuous Presence Multipoint and Video Multiplex (CPM), which is a multi-picture solution proposed by H.263 Standard. In this method, each of the sub-bitstreams encoded by the respective terminals involved in multi-picture combination is assigned with a unique indicator, which is abbreviated as “SBI” (Sub-Bitstream Indicator) used to distinguish between sub-bitstreams, and inserted into the picture header or the header of Group of Blocks (GOB) of the sub-bitstream.
In the second method, the MCU does not decode the video sub-bitstreams; instead, it only combines the sub-bitstreams in units of GOBs or frames to form a multi-channel composite bitstream; when receiving the multi-channel composite bitstream, the terminals distinguish between the respective channels of sub-bitstreams in the composite bitstream according to the SBIs in the picture headers or the GOB headers and then decode the channels of sub-bitstreams respectively, and finally display the pictures decoded from the sub-bitstreams in combination, thus to obtain the multi-picture.
However, the above methods in the prior have many drawbacks as follows:
Though the first method can implement combination of multiple sub-pictures, it is highly costly and consumes resources e.g. operation capability and memory severely at the MCU side. Though the second method costs less, the SBI can only represent four different values for a binary algorithm, as said SBI only consists of two bits as specified in syntax of H.263; therefore, the second method can only supports a combination of four channels of bitstreams at the most, i.e., a combination of four sub-pictures.